This invention relates to a frequency selective medium for selectively reflecting signals at a designated frequency band and for selectively transmitting signals at another designated frequency band, and more particularly, for selectively transmitting and reflecting microwave and millimeter wave signals with an angle of incidence that is other than normal.
Frequency selective media have been used for passing a designated band of frequencies while rejecting another designated band of frequencies. A conventional frequency selective medium for diplexing two frequency bands has been described in U.S. Pat. No. 5,162,809, which discloses an array of square or circular open center conductor elements deposited on a substrate. Although this frequency selective medium is suitable for passing certain designated frequency bands and rejecting other frequency bands for an incident microwave radiation at an angle normal to the surface or at a very small angle of incidence, it is not designed for frequency diplexing of incoming radiation at a large angle of incidence. Moreover, the ratio of transmitted microwave signal frequency to the reflected signal frequency is about 1.15, which means that the separation between the passband and stopband may be too large for some applications with stringent diplexing requirements. U.S. Pat. No. 5,373,302 describes another frequency selective medium for frequency division multiplexing in a dual reflector antenna, also known as a Cassegrain antenna. This frequency selective medium is also suitable for the frequency selection of an incident wave at a very small angle of incidence. At a relatively large angle of incidence, for example 45.degree., a significant frequency shifting of the passband and the stop band for the vertical and horizontal polarizations occurs in these conventional frequency selective media. Therefore, they are not suitable for the frequency selection of incoming radiation at a large angle of incidence such as 45.degree..
Other conductive surface structures for the transmission and reflection of microwave radiation have been theoretically described in Chao-Chun Chen, "Scattering by a Two-Dimensional Periodic Array of Conducting Plates," IEEE Transactions on Antennas and Propagation, volume AP-18, No. 5, September 1970, pages 660-665, and Chao-Chun Chen, "Transmission of Microwave Through Perforated Flat Plates of Finite Thickness," IEEE Transactions on Microwave Theory and Techniques, vol. MTT-21, No. Jan. 1, 1973, pages 1-6. These structures are not designed for microwave diplexing, that is, to pass a band of transmit frequencies and to reflect a stopband of rejection frequencies that are higher than the transmit frequencies. Another type of microwave surface structure is a meanderline polarizer, described in Leo Young, Lloyd A. Robinson and Colin A. Hacking, "Meander-Line Polarizer," IEEE Transactions on Antennas and Propagation, May 1973, pages 376-378. When linearly polarized microwave radiation impinges upon the meanderline polarizer, either a circularly polarized or a dual-linearly polarized wave with a 90.degree. phase difference emerges from the polarizer. These meanderline polarizers generally have a very wide passband and are not used for frequency diplexing.